The Pentax-A 50mm 1.7 will work with the *ist cameras. I don't have one, I have a M 50mm 1.7 instead. I don't think the optics are any different, so it should be a really sharp lens (mine is). Some people find it a bit soft at 1.7, but I haven't noticed it that much (now,my M 50mm 1.4 is quite soft at 1.4, OK at 1.7 and really nice beyond that). The advantage the A lenses give you is the ability for the camera to set the aperture, while the M lenses have to have the apeture set by the ring. Just my opinion, but I'm not sure I would pay a premium for that ability - I'd rather pay a premium for an F or FA lens.